1. Field of the Invention
The present invention relates to a method for fabricating a bonded SOI wafer, and more specifically, to a bonding process for the bonded SOI wafer.
2. Description of Related Art
In a conventional method for fabricating a bonded SOI wafer, as shown in FIG. 3, a single- or both-side mirror polished base wafer 1 and an active wafer 2, which has been single- or both-side mirror polished and thermal oxidization processed to form an insulating film (SiO.sub.2) 2a of a predetermined thickness are subjected to hydrophilic processing by cleaning them with the use of a solution of ammonia and hydrogen peroxide and then the wafers are pressed and bonded with the use of a bonding jig. And after passage of a predetermined period, the bonded wafers are subjected to a bonding thermal processing After bonding, the waiting period until the bonding thermal processing is suitably determined taking the due time and the availability of the furnace into account.
As stated above, the waiting period from the completion of bonding to the start of bonding thermal processing varies for each lot, and for some lots, as shown in FIG. 4, voids 3 are produced on the boundaries of the wafer. This figure shows a result of examination of the bonded SOI wafer by supersonic probes, which has been left for 36 hours after the bonding. The voids, which distribute in the region having a distance of 1.5 mm -5 mm to the wafer edge, tend to multiply as the waiting period from the completion of the bonding step to the start of the bonding thermal processing step is increased. The number of voids does not increase after the bonding thermal processing. If the bonded wafer has even only one void, this wafer must be discarded as a rejected article. Accordingly, the thermal processing must be adjusted so that the waiting period is shortened.
Conventionally, direct bonded wafers are used in which two silicon wafers are used as substitution for epitaxial wafers, for example. With respect to this bonded wafer, Japanese Patent Application Laid Open No. 7-263290 discloses a method for fabricating such a bonded wafer, in which two silicon wafers are bonded after cleaning them with hydrofluoric acid. In this method, since the oxide films on the surfaces of the wafers are removed, there has been a problem that an SOI wafer having an insulation film as an intermediate layer cannot be fabricated.